Some laser processes, such as welding, require an inert gas atmosphere, such as helium, argon, etc., about the area subjected to the laser beam, to prevent a chemical reaction of the material caused by the combined action of heat and atmospheric gases, of which oxygen is the most reactive.
In certain cases in which the material is not negatively affected by nitriding, dry nitrogen may also be used successfully.
On known devices for feeding such gas over the workpieces, the laser beam is directed and focused inside a conical nozzle together with a jet of inert shielding gas, usually with no separation of the two, and the inert gas is directed freely on to the work area.
Devices of the aforementioned type present numerous drawbacks. One of these is that the gas remains contacting a frequently excessive portion of the laser beam over what is usually an excessive length of time. In this respect, it is important to bear in mind that a gaseous mixture is heated by a laser beam in proportion to its capacity to absorb radiation at the laser beam wave length; in proportion to the length of time it is subjected to such radiation; and in inverse proportion to its heat capacity. The first undesired effect of such heating is that it results in an irregular alteration of the refraction coefficient of the gas, thus distorting and impairing the focus of the laser beam, which is further impaired by the convective motion produced in the heated mixture.
Should heating persist long enough for the gas to reach first the thermal excitation and then the thermal ionization threshold, this results in highly dissipative phenomena, which absorb the energy of the laser beam. In extreme cases of almost total or predominant ionization, the energy of the beam may be almost totally absorbed by the gas, thus giving rise to what is known as "blanketing", whereby the laser beam is practically prevented from reaching the workpiece.
The shielding gas must therefore present a definite number of properties:
1) low radiation absorption coefficient at the laser beam wave length; PA1 2) high heat capacity; PA1 3) high thermal ionization threshold or ionization potential; PA1 4) minimum interaction or transit time (the length of time it is subjected to the laser beam); PA1 5) for shielding against thermochemical reactions, the gas must be immune to such reactions. For this, noble gases are preferred, especially helium (He), the ionization potential and heat capacity of which are among the highest, and which also presents a low absorption coefficient of the most commonly used industrial laser beams. PA1 Po+.DELTA.P&gt;&gt;Po and To+.DELTA.T&gt;&gt;To PA1 a) undesired, totally irregular stress; PA1 b) severe distortion; PA1 c) poor process efficiency; PA1 d) poor process repeatability; PA1 e) unsatisfactory correlation with available models.
In view of the above considerations, the most logical choice is helium, providing it is economically feasible, especially in Europe.
Another possibility is argon. In addition, however, to a higher radiation absorption coefficient at the wave lengths commonly used in industry, this also presents a low heat capacity and ionization potential as compared with helium. These drawbacks, however, may be overcome by proportionally reducing the interaction time, and by appropriate streamlining of the shielding gas jet.
Moreover, as ionization is predominantly thermal and, as such, governed by Saha's law, steps may also be taken to increase the pressure and reduce the temperature of the gas jet in the laser beam crossover region.
One of the major objectives involved, therefore, is that of achieving a uniform gas jet, the properties of which remain unchanged throughout its crossing of the laser beam, i.e. uniform refraction and parallel motion, no convective motion or excitation, and no ionization.
Moreover, the jet must present a poor thermophotochemical reaction with the shielded material. (Hence, no water, oxygen, hydrocarbons, acids, salts, alkalis, etc.).
The impact of the shielding gas, in the form of plasma, on the surface of the workpiece has been found to cause a considerable thermal alteration of the surface, accompanied by vapourization of a layer of material on either side of the weld bead. In particular, in cases where the gas jet is used firstly for cooling a lens, and is then directed, together with and penetrating the length of the laser beam, into a conical nozzle, the issuing jet is substantially ionized. What is more, being directed perpendicularly to the surface of the workpiece, upon impact, the jet not only reaches the high temperatures and pressures corresponding to impact velocity 0 (zero), but is also forced to restore the ionization energy by recombining it.
By combining these effects (high Po, high To) plus the enthalpic values corresponding to restoration of the ionization energy, i.e.
it follows that the material of the workpiece vapourizes, not beneath the laser beam, but at the edge where it is struck by the plasma jet.
Another point to note is that the high reflectivity of a metal surface at low temperature (practically until it becomes red hot) almost doubles the intensity of the laser beam on impact, thus resulting in ionization despite the low value of the incident laser beam.
This is further assisted by gas, free radicals and other adsorbed chemically active substances, which emit nonthermionic free electrons resulting in avalanching, and, if serious enough, even in blanketing, i.e. an unpredictable (catastrophic) increase in ionization at incident laser beam level, which prevents transmission of the beam on to the workpiece.
Ionization is always accentuated by, and very often in fact due to, the surface of the workpiece, which is inevitably reflective at low temperature, and therefore reflects part of the energy of the laser beam back towards the inert gas atmosphere, thus roughly doubling the intensity of the radiant energy field in the area close to the work surface.
If, on the other hand, the inert gas jet is directed on to the work surface together with the laser beam, it presents a high degree of ionization on impact with the workpiece, and, on bouncing back off the work surface, tends to mix with the atmosphere, thus reducing shielding efficiency. Moreover, ionization energy is taken from the laser beam, which therefore presents a lower power density for a given focal spot; and the variable density of the mixture so formed results in refraction and convection phenomena impairing the focus of the laser beam, thus further reducing its intensity and efficiency.
According to the known state of the art, the jet is supplied freely, often even using conical nozzles. Consequently, when supply pressure increases over and above a critical ratio value (corresponding to sonic velocity at the cone outlet, and characterised by an (outlet pressure)/(stagnation pressure) value of &lt;0.528 for biatomic gases such as air, nitrogen, oxygen, carbon monoxide, etc.), the jet, for lack of a stable law of motion, oscillates violently in direction (wobble) and axial velocity (pulsation). These effects amplify those caused by air mix and drag, and by possible laminar or turbulent boundary layers established inside the nozzle and affecting uniform outflow. In view of the size and distance of commonly used nozzles from the work surface, this combined alteration in outflow, due to the internal and, even more so, external aerodynamic factors involved (boundary layer, air mix and drag), is even more serious in the event oscillation (wobble, pulsation) is amplified by attempts to increase outflow velocity by increasing supply pressure. In the case of normal size subsonic nozzles, the instability caused by exceeding the critical pressure frequently results in such severe oscillation that the laser beam impact area is substantially uncovered, thus impairing shielding by the gas, which may be replaced by varying mixtures of air and gas. These alternate over the impact point of the laser beam, which is thus swept by a pulsating jet in which the percentage of shielding gas may vary enormously and in a random manner characteristic of this type of non-stationary phenomena.
Reliable control of a freely directed jet requires the use of supersonic nozzles, which, with a given configuration and pressure ratio (obviously, for a given type of gas or mixture), provide for a uniform jet. If the pressure ratio is other than nominal, however, this results in the formation, at the outlet, of what is known as Prandlt diamonds and Mach disks. Freely directed supersonic jets nevertheless provide for a better distribution of the field parameters (velocity, density, pressure, temperature), and, though subject to the above internal and external aerodynamic factors (boundary layers, air mix and drag), these are less marked and controllable.
In this case also, scale effects, expressed in Reynolds, Prandlt, Nusselt numbers, etc., must obviously be taken into account. The capacities, distances and sizes used in laser applications may result in field irregularities (the properties of the jet in terms of its parameters, including the incorporation of outside air) possibly affecting shielding efficiency in the laser beam-work surface impact area. Nevertheless, supersonic nozzles provide for achieving velocities and pressures otherwise unattainable using subsonic nozzles.
The latter, very often, are not only badly designed, but also employed with no regard whatsoever to the basic laws of aerodynamics.
Consequently, when the stagnation pressure of a freely directed jet, particularly a subsonic jet, is increased with a view to increasing crossover speed, ambient gas drag efficiency, and also surface pressure, for improving shielding efficiency, this more often than not results in uncontrollable situations the effects of which are quite the opposite to those expected. The commonest include uncontrolled ionization resulting in "nailheads" (irregular weld bead); vapourization and sublimation of the surface material, which often presents grooves on either side of the weld bead; and an irregular metallurgical structure, which often presents a central zone, and two surrounding zones reasonably attributable to the plasma jets on either side of the laser beam responsible for the central zone. This differs widely from the two surrounding zones, though all three are irregular. Structural and geometric irregularity clearly indicates, among other things:
The above introduction clearly indicates, therefore, the importance of correct utilization of the shielding gas, for improving the process in terms of efficiency, quality and repeatability.
Correct usage will also provide for a better correlation of the data involved, thus enabling a better understanding of the phenomenon and its control parameters, and more straightforward, effective models for predicting, achieving and maintaining the required results.
GB-A-2 045 141 discloses a method and apparatus for the control of shielding gas according to the preamble of claims 1 and 7, respectively, wherein a jet of shielding gas is directed from a nozzle across the worpiece surface and recirculated to the nozzle along a ducting, of which the nozzle defines an output.
The input of the circulation duct is spaced apart with respect to the nozzle, the shielding gas being therefore not guided within solid walls in the area of interaction with the laser beam and not separated from the ambient air. Even if the circulation duct is so well designed as to theoretically remove all the shielding gas erected by the nozzle, severe heating by laser beam in the interaction area, where a free boundary layer between such gas and ambient air exists, causes the flow to become inevitably unsteady, and the shielding gas to be mixed with ambient air, thus producing the drawbacks previously discussed, that is: recirculation and thereby long residence time resulting in heating and ionization of the easily ionizable gas mixture. FR-A-2 360 376 discloses a gas shielding apparatus wherein the shielding gas is fed to a cavity surrounding the welding area through a narrow passage. With this geometrical arrangement, supercritical flow conditions are easily reached; oscillations in flow speed and direction at the outlet of the passage are to be expected, and outflow irregularities are further amplified by laser heating, so that a random gas recirculation at low average speed in the cavity is established, which leads to gas ionization and laser beam defocusing.
It is an object of the present invention to provide a straightforward, reliable gas shielding method and device designed to overcome the aforementioned drawbacks.